The present invention relates to optical scanning devices that can be applied in image forming apparatuses such digital copiers, printers, and fax machines, and to image forming apparatuses.
Conventionally this type of optical scanning device has been installed in image forming apparatuses such as digital copiers, printers, and fax machines, and has been widely used as an optical writing means in these image forming apparatuses.
In these image forming apparatuses provided with optical scanning devices, in a case where image forming is to be performed using an electrophotographic image forming process for example, a surface of an image carrier (a photosensitive body or the like for example) acting as a scanning object is charged, then on the charged area is formed (written) an electrostatic latent image by irradiating an optical beam from a light source in the optical scanning device onto a rotating multifaceted mirror (also referred to as a polygonal mirror) while modulating the optical beam based on image information so as to scan the surface of the photosensitive body in a scanning direction.
The optical scanning device irradiates the optical beam from the light source toward the rotating multifaceted mirror having a plurality of reflective surfaces that are arranged along a rotation direction around the rotational axis, then an incoming beam that is incident from the light source onto the reflective surfaces of the rotating multifaceted mirror is reflected (becomes outgoing) by the reflective surfaces of the rotating multifaceted mirror, and a scanning surface on the scanning object is scanned by the outgoing beams reflected by the reflective surfaces of the rotating multifaceted mirror.
Broadly divided, two types of optical systems are known in optical scanning devices, one being an underfilled optical system in which the optical beam from the light source is irradiated onto only a portion of the reflective surfaces of the rotating multifaceted mirror, and the other being an overfilled optical system, in which the optical beam from the light source is formed wider than a width of the reflective surfaces of the rotating multifaceted mirror in the rotation direction to be irradiated onto the reflective surfaces of the rotating multifaceted mirror.
Of these, a configuration is common in optical scanning devices provided with the overfilled optical system in which, due to the nature of the optical beam from the light source being formed wider than the width of the reflective surfaces of the rotating multifaceted mirror in the rotation direction to be irradiated onto the reflective surfaces of the rotating multifaceted mirror, the incoming beam is incident on the reflective surfaces of the rotating multifaceted mirror in a state parallel to a virtual vertical surface that is vertical to the scanning surface, and vertical to the scanning direction of the outgoing beams.
The following problems exist in conventional optical scanning devices comprising an overfilled optical system.
FIG. 17 is an schematic view showing a conventional optical scanning device comprising an overfilled optical system.
The optical scanning device shown in FIG. 17 is provided with a light source 311d, a collimator lens 312d, a concave lens 313d, an aperture plate 314d, a cylindrical lens 315d, a folding mirror 316d, a rotating multifaceted mirror 320d, an f-theta lens 331d, a cylindrical lens 332d, and a scanning object 21 such as an image carrier.
In the optical scanning device shown in FIG. 17, an optical beam L from the light source 311d is reflected by the folding mirror 316d to form an incoming beam Li that is wider than the width of a reflective surface 320a of the rotating multifaceted mirror 320d in the rotation direction (a clockwise direction Z in the example of FIG. 17), and the thus-formed incoming beam Li is incident on the reflective surface 320a of the rotating multifaceted mirror 320d and a scanning surface 21a of the scanning object 21 is scanned by an outgoing beam Lo reflected by the reflective surface 320a. At this time, the incoming beam Li is incident on the reflective surface 320a of the rotating multifaceted mirror 320d in a state parallel to a virtual vertical surface α that is vertical to the scanning surface 21a and vertical to a scanning direction X of the outgoing beam Lo.
FIG. 18 is a graph showing a light amount (light intensity) distribution β0 with respect to a width direction H distance (a distance from a center C of the incoming beam Li) of the incoming beam Li that is incident from the light source 311d onto the reflective surface 320a of the rotating multifaceted mirror 320d. 
As shown in FIG. 18, the incoming beam Li, which is incident from the light source 311d onto the reflective surface 320a of the rotating multifaceted mirror 320d, has a strongest light intensity at the center C of the width direction H, and indicates a normal distribution (Gaussian distribution) that progressively diminishes toward both outer sides from the center C of the width direction H.
When the incoming beam Li shown in FIG. 18 is incident on the reflective surface 320a of the rotating multifaceted mirror 320d in an optical scanning device comprising an overfilled optical system as shown in FIG. 17, the light amount becomes undesirably uneven, regardless of the necessity for the light amount to be even at every scanning position of the scanning surface 21a in the scanning direction X, when the scanning surface 21a is scanned by the outgoing beam Lo that is outgoing from the reflective surface 320a. 
FIGS. 19(a) to 19(c) are descriptive diagrams for describing how the light amount becomes uneven in a scanning region R in the scanning direction X of the scanning surface 21a in the optical scanning device shown in FIG. 17, and show both the incoming state of the incoming beam Li and the outgoing state of the outgoing beam Lo with respect to the rotating multifaceted mirror 320d, together with the light amount distribution β0 with respect to the width direction H distance of the incoming beam Li. FIG. 19(a) shows a state in which the outgoing beam Lo is outgoing from the reflective surface 320a of the rotating multifaceted mirror 320d to an upstream side of the scanning direction X with reference to the virtual vertical surface α. FIG. 19(b) shows a state in which the outgoing beam Lo is outgoing from the reflective surface 320a of the rotating multifaceted mirror 320d and parallel to the virtual vertical surface α. FIG. 19(c) shows a state in which the outgoing beam Lo is outgoing from the reflective surface 320a of the rotating multifaceted mirror 320d to a downstream side of the scanning direction X with reference to the virtual vertical surface α.
Here, in regions β1, β2, and β3 corresponding to the outgoing beam Lo of the incoming beam Li, the light intensity of the outgoing beam Lo becomes greater toward the center of the incoming beam Li. Furthermore, the light intensity of the outgoing beam Lo is greater for larger beam widths h (incoming beam surface area) corresponding to the outgoing beam Lo of the incoming beam Li. Further still, a reflectance of the incoming beam Li by the reflective surface 320a, which is a ratio of the outgoing beam Lo to the incoming beam Li, is larger for smaller incidence angles φ of the incoming beam Li to the reflective surface 320a. 
From these characteristics, when the outgoing beam Lo is outgoing from the reflective surface 320a of the rotating multifaceted mirror 320d to the upstream side (left side in the diagram) of the scanning direction X with reference to the virtual vertical surface α as shown in FIG. 19(a), the light intensity becomes stronger closer to the virtual vertical surface α in the region β1 corresponding to the outgoing beam Lo of the incoming beam Li. Moreover, in this state, an incidence angle φ1 becomes smaller as the outgoing beam Lo becomes closer to the virtual vertical surface α. Furthermore, a beam width h1 (incoming beam surface area) corresponding to the outgoing beam Lo of the incoming beam Li which is incident on the reflective surface 320a for the scanning object becomes larger as the outgoing beam Lo becomes closer to the virtual vertical surface α.
In contrast to this, when the outgoing beam Lo is outgoing from the reflective surface 320a of the rotating multifaceted mirror 320d parallel to the virtual vertical surface α as shown in FIG. 19(b), the light intensity is strongest in a region β2 corresponding to the outgoing beam Lo of the incoming beam Li, and moreover a beam width h2 (incoming beam surface area) corresponding to the outgoing beam Lo of the incoming beam Li is larger compared to the state shown in FIG. 19(a).
On the other hand, when the outgoing beam Lo is outgoing from the reflective surface 320a of the rotating multifaceted mirror 320d to the other side (right side in the diagram) of the scanning direction X with reference to the virtual vertical surface α as shown in FIG. 19(c), the light intensity becomes weaker farther from the virtual vertical surface α in a region β3 corresponding to the outgoing beam Lo of the incoming beam Li. Moreover, in this state, an incidence angle φ3 becomes larger as the outgoing beam Lo becomes farther from the virtual vertical surface α. Furthermore, a beam width h3 (incoming beam surface area) corresponding to the outgoing beam Lo of the incoming beam Li which is incident on the reflective surface 320a for the scanning object becomes smaller as the outgoing beam Lo becomes farther from the virtual vertical surface α.
FIG. 20 is a graph showing a light amount (light intensity) distribution γd with respect to a scanning direction X distance on the scanning surface 21a of the scanning object 21 (distance from a scanning position R0 where the outgoing beam Lo becomes parallel to the virtual vertical surface α) in the optical scanning device shown in FIG. 17.
As shown in FIG. 20, in the scanning region R of the outgoing beam Lo on the scanning surface 21a, in the state shown in FIG. 19(a), the light amount (light intensity) exhibits a tendency to increase progressively as the outgoing beam Lo becomes closer to the scanning position R0 where the outgoing beam Lo becomes parallel to the virtual vertical surface α as shown by a reference symbol γ1 in FIG. 20. Furthermore, in the state shown in FIG. 19(b), in the scanning region R of the outgoing beam Lo on the scanning surface 21a, the light amount (light intensity) is strongest at the scanning position R0 as shown by the reference symbol γ2 in FIG. 20. Furthermore, in the state shown in FIG. 19(c), in the scanning region R of the outgoing beam Lo on the scanning surface 21a, the light amount (light intensity) exhibits a tendency to decrease progressively farther from the scanning position R0 as shown by the reference symbol γ3 in FIG. 20.
That is, the light amount distribution γd on the scanning surface 21a in the scanning direction X is mountain shaped.
These facts can be summarized as shown in Table 1 below.
TABLE 1 small since edgelargest sincesmall since edgepower ofof incidentcenter of incidentof incidentincoming beamdistribution useddistribution useddistribution usedwidth ofmedium (h1)large (h2)medium (h3)incoming beamincidence angle medium (mediumnonemediumφreflectance)(largereflectance)(mediumreflectance)power ofmediumlargemediumoutgoing beam
It should be noted that these facts are the same for a case where the rotating multifaceted mirror 320d rotates in a reverse direction (counterclockwise direction in the examples of the drawings).
In relation to this, JP 2003-322816A discloses an example in an overfilled scanning optical system in which a filter is used as a diffractive optical element through which light from the light source is transmitted, with the filter exhibiting an optical transmissivity distribution of a substantially opposite shape to the mountain shaped light amount distribution in the scanning direction on the image surface (scanning surface).
In this overfilled scanning optical system, since the filter is used exhibiting an optical transmissivity distribution of the opposite shape (curved shape) to the mountain shaped light amount distribution in the scanning direction on the image surface (scanning surface), the filter has to be designed and manufactured to achieve a curved shaped optical transmissivity distribution aligned with slight changes in the light amount distribution, which correspondingly reduces leeway in the filter design and complicates the manufacturing of the filter, and thus there is a problem that the design and manufacture of the optical system is complicated. Generally, it is common for these filters to involve using a deposition method to form a light-shielding material as a film onto an optically transmissive member such as a glass plate, and in this case it is insufficient to improve the light amount distribution of the mountain shape, and unfortunately time and effort are required to manufacture a filter that exhibits the optical transmissivity distribution of the curved shape.
For example, when forming a film on the optically transmissive member, even though measures are attempted such as depositing the light-shielding material onto the optically transmissive member in the curved shape, in this case the manufacture of the filter will require excessive time and effort.
To address this issue, it is common for example to employ a filter that exhibits an optical transmissivity distribution in which the optical transmissivity is distributed partially different in the scanning direction.
For example, JP 2003-287694A discloses an example in which, as a filter that transmits light from the light source in an overfilled optical system, a filter is used that exhibits an optical transmissivity distribution in which the optical transmissivity of a central area in the scanning direction is extremely lowered and the optical transmissivity of the edge portions is extremely raised.
This overfilled optical system aims to achieve light amount uniformity on the scanning surface by controlling the optical transmissivity partially using the filter that exhibits a distribution in which the optical transmissivity is partially different in the scanning direction, but this is merely maintaining a uniform light amount macroscopically, and when viewed microscopically, the light amounts vary greatly at scanning positions corresponding to borders between regions having a lower optical transmissivity and regions having a higher optical transmissivity, and therefore, for example, this incurs changes in the image density obtained by the image forming apparatus at scanning positions corresponding to these borders.
For example, in a case of using deposition to form a light-shielding material as a film onto an optically transmissive member such as a glass plate, even with attempts at improvement such as arranging a mask portion diagonally to mask the optically transmissive member from the light-shielding material when forming the film on the optically transmissive member, it is a fact that positions (inflection points) still remain where the light amount varies greatly.
Furthermore, JP H6-214184A discloses an example in which, in an overfilled raster scanning system, a binary diffractive optical lens system including two diffractive optical lens elements is used to re-profile the optical beam intensities from the light source and generate a profile having a uniform intensity.
In this overfilled raster scanning system, the diffractive optical lens elements have to be designed and manufactured to achieve a profile having a uniform intensity by re-profiling the optical beam intensities from the light source, which correspondingly reduces leeway in the design of the diffractive optical lens elements and complicates the manufacturing of the diffractive optical lens elements, and thus there is a problem that the design and manufacture of the optical system is complicated.
Furthermore, to achieve uniform light amounts at scanning positions in the scanning direction on the scanning surface, it is also conceivable to control the light emission amounts of the light source aligned with slight changes in the light amount distribution for scanning direction positions on the scanning surface when scanning the scanning surface using the outgoing beam, but the control configuration becomes correspondingly complicated when controlling the light emission amounts of the light source aligned with slight changes in the light amount distribution, and thus the design of the optical system becomes complicated.